Yes, we know, you've been reminded approximately a bazillion times to wash your hands regularly during cold and flu season. But quick refresher on why it's so, so important: Virus-containing droplets expelled via sneezes or coughs can easily be transferred between people—even by simply shaking hands or grabbing a doorknob and then touching your nose or mouth.

But while a scrub with soap and water is your safest bet in those instances (after a flu shot, of course), a sink isn't always readily available; sometimes you just can't pry yourself away from your desk, or you're in the middle of an outdoor workout. “You can’t just be in the bathroom washing your hands all day,” says Pritish K. Tosh, MD, a Mayo Clinic infectious disease physician and researcher.

Enter hand sanitizer. The alcohol-based gel plays the role of knight in shining armor for many of us who can’t stop what we’re doing to scrub-a-dub-dub around the clock. “Because of their convenience and efficacy, using them seems like a really good idea,” Dr. Tosh agrees.

And hand sanitizer often is a good idea—as long as you follow a few simple rules.

Clean your hands with hand sanitizer when soap and water aren't available

Washing your hands with soap and water is still your first line of defense against a host of illness-inducing organisms, Dr. Tosh says. But when you can’t make it to the sink, hand sanitizer can fight those bugs too, including viruses that cause colds and the flu.

However, a study published this week in the journal Pediatrics raises questions about situations in which hand sanitizer could be more effective than washing up. The research found that and miss daycare when they used hand sanitizer compared to when they washed their hands.

“Because it’s often easier to use hand sanitizers, people may be more likely to use it and do it more often than if somebody were just to stick with soap and water,” Dr. Tosh hypothesizes. “Even if the efficacy [of hand sanitizer] may be lower, it’s overall ability to prevent infection may be greater because it’s easier to do more often.” Still, he says, experts aren't giving us the green light to skip the sink entirely.

Don't use hand sanitizer when your hands are visibly dirty

According to the Centers for Disease Control and Prevention (CDC), sanitizers simply —say, after you’ve been gardening or tinkering with your bike gears.

For starters, that dirt and grease won't go away if you're just adding sanitizer to the mix, says Tanaya Bhowmick, MD, assistant professor of medicine in infectious diseases at Robert Wood Johnson Medical School in New Jersey. “If you have dirt on your hands and you put alcohol on it, you’re really just creating a slurry.” You’ll rub that gunk around without actually washing it off, she explains.

And because hand sanitizer doesn’t kill every microbe, there are some you really do need to wash off, she emphasizes.

Make sure your sanitizer is at least 60% alcohol

The alcohol acts as what’s called a denaturing agent, Dr. Tosh explains, versus soap, which acts as a detergent. Essentially, alcohol kills or inactivates viruses—and it does so most effectively in sanitizers that are between 60% and 95% alcohol, according to the CDC.

Proper application is important too, Dr. Bhowmick adds. Apply hand sanitizer to the palm of one hand, then “keep rubbing all over your hands until it’s dry,” she says. Friendly reminder: You shouldn’t be wiping off your hand sanitizer, whether you’re using a towel or the legs of your jeans (hey, we've been there). “It defeats the purpose because whatever you’re wiping it off on, you could be picking something else up off that,” Dr. Bhowmick says.

We like ($10, ) and with aloe ($13 for 4, ). Looking for an alcohol-based sanitizer with some moisturizing agent like aloe is a smart move, Dr. Tosh says, since all that alcohol can be drying.

Avoid anything labeled "antibacterial"

If you're an obsessive hand sanitizer user, you may have wondered if you're using it too much. Luckily, alcohol-based gels should continue to work just as well over time, so keep on rubbing on. “Up until now at least, there’s no data showing it’s not as effective [over time],” Dr. Bhowmick says—when it comes to killing viruses, at least. There is some research suggesting might develop a tolerance to alcohol though, she says.

That's somewhat concerning, considering the ever-growing threat of microbial resistance—when bacteria evolve to survive the meds typically used to kill them. Overusing antibacterial and antimicrobial products can bolster those so-called superbugs, Dr. Tosh says, so stay away from hand gels boasting those proprieties on their labels.